The present invention relates to a packaging structure and method, more particularly, to a packaging structure and method utilizing a thixotropic silicone gel.
Silicone gel is commonly used fur protection of electronic parts and connecting wires in automotive components, such as, engine control units and sensor units due to silicone gel""s low elastic modulus and its ability to greatly reduce thermal stress resulting from differences among components coefficient of thermal expansion. Also, silicone gels have good heat resistance and are resistant to changes in physical properties in temperatures, particularly, from about xe2x88x9255 to 150xc2x0 C. Further, silicone gels have low surface tension, good wettability and adhesion.
For instance, a conventional automotive engine control unit is schematically shown in FIG. 3. On ceramic base 1 are mounted chip parts 3 (such as capacitors and resistors) and a semiconductor bare chip 4. Then, they are connected to the surface conductor of the ceramic base by wire bonding 5 or soldering. The ceramic base is mounted on and bonded to a metal radiator 6 which is integral with a plastic or metal connector 7 as a casing.
For electrical communication, the connector""s terminal leads are connected to pads on the ceramic base by means of wire bonding 5, soldering, or TAB (tape automated bonding). The assembly is coated with a silicone gel 18, which is subsequently heated, for the protection of electronic parts against moisture and foreign matter in the atmosphere. Finally, the casing is hermetically closed with a cap 11 by means of an adhesive or rubber O-ring.
A conventional automotive pressure sensor is schematically shown in FIG. 4. On a glass base 12 is mounted a sensor chip. They are hermetically bonded together in a vacuum by means of soldering or anodic process. The glass base 12 is bonded with an adhesive to the inner bottom of a resin casing 15 having external terminals. The sensor chip is connected to the external terminals through bonded metal wires 16. The surface of the sensor chip and metal wire is sealed with a silicone gel 18 which is poured into the resin casing 15. Resin casing 15 is bonded with an adhesive cap 11 having a pressure port 17. As in the case of automotive engine control unit of FIG. 3, the silicone gel 18 protects the sensor chip 13 and connecting metal wires 16 from the atmosphere.
However, silicone gels can be problematic. For instance, while a low-viscosity silicone gel permits efficient application to the surface of ceramic bases, it needs a frame or dam to prevent it from spreading out unnecessarily. Conventionally, the resin or metal casing functions as a frame or dam to prevent the silicone gel from spreading out. However, since the silicone gel does spread out, it is necessary to pour a large amount of it to cover the thick electronic parts mounted on the base. This changes the characteristic properties of the control unit or sensor and makes it necessary to remove the excess silicone gel. Moreover, the excess silicone gel can adversely affect the bonding of a cap, causing peeling and cracking.
However, a thixotropic silicone gel retains its shape after application to the surface of the ceramic base and seals only those parts to be sealed. But, a conventional thixotropic silicone gel rapidly increases in viscosity, causing deteriorating coatability and voids. Moreover, it has a poor ability to impregnate into narrow interstices between closely packed parts or wires.
The present invention provides a packaging applied to a highly reliable automotive component, such as, a engine control unit or sensor, having a thixotropic silicone gel superior in workability owing to its physical properties within a specific range.
An object of the present invention provides a packaging structure applied to an automotive component to regulate the engine and transmission, having semiconductors and electronic parts mounted on a ceramic base, characterized in that the semiconductors and electronic parts are partly or entirely sealed with a thixotropic silicone gel which has a thixotropy index of about 1.5-3.6 and a penetration depth of about 6-10 mm, and a rate of change in viscosity of no more than 10% of the initial value.
In a preferred embodiment, the above-mentioned thixotropic silicone gel should additionally have a viscosity of 1000-5000 mPaxc2x7s at 25xc2x0 C., a light transmittance greater than 60%, an exothermic peak at 100-140xc2x0 C., a one-pack form of uniform mixture and a rate of change in viscosity of no more than 10% of the initial value after 7 days at 40xc2x0 C.
In the above-mentioned packaging structure for an automotive component, the ceramic base can be mounted on a heat radiating metal base, for instance, an aluminum base. Preferably, the thixotropic gel has a thickness of about 10-1000 xcexcm. The packaging structure, from the bottom of the metal base to the top of the package structure is about 3-10 mm high.
Another object of the present invention provides a packaging method applied to an automotive component composed of a ceramic base and semiconductors and electronic parts mounted on said ceramic base, said method comprising coating said semiconductors and electronic parts partly or entirely with a thixotropic silicone of the present invention and heat-hardening the silicone gel layer covering said ceramic base, semiconductors, and electronic parts.
Another object of the present invention provides a packaging structure applied to an automotive component having semiconductors and electronic parts mounted on a ceramic base, said semiconductors and electronic parts being connected by a conductive line, preferably a metal wire, bonding to the external terminal leads of a connector, characterized in that the uppermost part of the terminal leads of the connector is equal to or shorter in height than the uppermost level of said ceramic base, and said semiconductors and electronic parts mounted on said ceramic base are sealed with a thixotropic silicone of the present invention.
Alternatively, the terminal leads of the connector are constructed in at least two tiers and the uppermost part of the terminal leads of the connector is equal to or shorter in height than the uppermost level of said ceramic base, and the semiconductors and electronic parts mounted on the ceramic base are sealed with a thixotropic silicone gel of the present invention.
In yet another embodiment of the present invention, the casing has a main lower surface which is a heat-radiating metal base and has an upper surface which has a ceramic base attached thereto, the ceramic base has an outer frame which supports semiconductors and electronic parts, the outer frame being 3-10 mm high from the bottom of the heat-radiating metal base, and the semiconductors and electronic parts mounted on the ceramic base are partly or entirely sealed with a thixotropic silicone gel of the present invention to a thickness of about 10-1000 xcexcm.